Generally, this type of meter rotates a pointer fixed to one end of a drive shaft of the movement of the meter corresponding to an input signal and indicates a measurement by way of contrast and legibility against a dial provided with numbers or a scale for showing a measurement amount, and is normally constructed so as to indicate a linear indication characteristic by a scale of substantially equal increments.
In particular, in the case of the stepping motor the stepping operation of the magnet rotor is determined by the number of tooth of the toothed yoke and the pitch formed thereby so that in order to attain a smooth rotating operation, it is necessary to increase the number of teeth and reduce the pitch of the teeth, or alternatively, perform a so-called micro-step drive by means of a drive signal and the type of stepping motor is selected based on the allowable size of the stepping motor main unit meeting the condition of use and the cost of including a drive circuit.
Further, such a stepping motor as this is desired to be compact regardless of its applications. Since a so-called PM type stepping motor has a simple structure, it is made easy to use due to its improved rotor magnet and toothed yoke.
Further, in contrast to the digitization of the processing circuit (controlled by a microcomputer), this type of stepping motor in which pulse signal control is performed has attracted attention also as a movement for an indicating meter, which indicates a comparative readout on a dial scale by means of a pointer--for example, it can be used in a speedometer or engine tachometer of an automobile, or as a heat gauge or thermometer by way of A/D processing of a detection signal, with various proposals for practical applications forthcoming, such as those disclosed in Unexamined Japanese Patent Application S61-129575, Unexamined Japanese Patent Application H1-223312, etc.
In addition, it is common for a plurality of indicating meters such as a speedometer or tachometer to be used simultaneously in this type of measuring apparatus, and in cases where a processing circuit is provided for each of these meters it is necessary to use the least expensive circuit components in view of the production cost, to which extent indication control having a slow computing speed becomes inevitable.
Also, with the digitization of signal processing, time division processing by means of a digital processing circuit such as a microcomputer from a control system in which each indicating meter is processed is developing, wherein the computation processing cycle is allocated by the responsiveness of each meter corresponding to the subject being measured (speedometers and tachometers have fast cycles, heat gauges and thermometers have slow cycles), necessitating processing circuits with high computation processing speeds according to an increase in the number of driving indicating meters.
However, where expensive processing circuits are not used in view of the production cost, or where many indicating meters are time-divisionally driven, there is a limitation to the compression of a computation processing cycle allocated to one indicating meter. For example, given that a drive update cycle necessary to smoothly rotate the pointer corresponding to a measurement amount which varies to a great degree, such as engine revolutions, is 10 milliseconds, and the allocation limit of the computation processing cycle of the processing circuit is only 20 milliseconds, the pointer can only move intermittently with respect to changes in the measurement amount and a smooth indication characteristic cannot be attained.
In particular, although a stepping motor attains smoothness by correcting the wave form of the drive signal on the basis of a step operation corresponding to the pitch of the teeth on the toothed yoke, where a measurement data updating cycle is large with respect to changes in a measurement amount, changes in the drive signal to the stepping motor itself are also large. Also, although when changes in the measurement amount are fast the intermittent movement of the pointer is relatively inconspicuous due to its fast movement, where the responsiveness of the stepping motor itself is excellent or, conversely, where changes in the measurement amount are gradual, it was confirmed that large changes in this data directly caused intermittent movement of the pointer and there was the problem that a smooth indicating characteristic as an indicating meter could not be achieved.
The present invention has for its object to enable smooth drive to an extent which is imperceptible even where the conversion processing capability, i.e. conversion processing cycle, of a processing circuit for computation processing measurement data and converting and outputting an indication angle signal is not sufficiently short to attain smooth movement of the stepping motor with respect to changes in the measurement amount.